As readers of Keats will recognize, the last line here is a conscious echo of the close of "To Autumn": "And gathering swallows twitter in the skies."

It's the "gathering" that gets me -- the end product of some final work in draft, Keats first trying "And now flock still...", then amending that to begin the line again with the participial "Then gathering..."

I've wondered if among the many beloved lines filed in his poetic memory he was here recalling Virgil's image of swallows as souls gathered on the shores of the underworld, awaiting emigration. Then too, Robert Gittings suggests an echo of Dante's calling cranes and massed starlings, from Inferno VI. 46-48. Both these passages Keats surely knew.

My own private association here brings up Gilbert White, who, writing in Selborne, not too far away in either time or place from Keats's compositional location (Winchester, early autumn, 1819), remarked on Sept. 7, 1791 of the intimations bespoken in the gathering of hirundines prior to migration: "Such an assemblage is very beautiful, & amusing, did it not bring with it an association of ideas tending to make us reflect that winter is approaching, & that these little birds are consulting how they may avoid it."

(Incidentally, James Thomson's "Autumn", which Keats also knew, has gathering swallows twittering: perhaps also part of the ambient word-flock or as we might now say, tag-cloud, of a poet's imagination.)

Barbara Everett, one of the most sensitive of Keatsian critics, writes of the last line of "To Autumn", "The present participle becomes the most exquisite, disturbing word in the whole serene poem because of all that it communicates of incipience in time, the survival of selves beyond the present self."

Ah, Gilbert White, the great naturalist, observer of life and most wonderful of diarists!

Yes, his phrasing is always so perfect, and "pure" as well. I love "consult" in this context, it makes the hirundines into a very serious congregation indeed, the history of the verb carrying one back to the convening of the Roman Senate, "to call together" (to ask for advice, & c.).

GW never lets us forget that Nature is no less considerate in its workings than we would take ourselves to be.

(He is one of those few writers whom it may well be sacrilegious to read online, but if one has no other way, much of his keeping of the natural calendar of Selborne may be found at Project Gutenberg and I believe at some biological/botanical/ornithological heritage sites as well.)